Material and technique fundamentals of nano-hydroxyapatite coatings towards biofunctionalization: a review

Hydroxyapatite (HAp) nanocoatings on titanium alloys (e.g. Ti6Al4V) have been used for prosthetic orthopaedic implants in recent decades because of their osseointegration, bioactivity, and biocompatibility. HAp is brittle with low mechanical strength and poor adhesion to metallic surfaces, which limits its durability and bioactivity. Surface modification techniques have alleviated the imperfections in biomaterials by coating the substrate. Several methods for improving the characteristics of implants, such as physical vapour deposition, the thermal spray method, the sol-gel method, microarc oxidation, and electrochemical deposition methods, have been discussed in this review. These processes provide mechanical strength without sacrificing biocompatibility and may lead to the development of new ideas for future research. This review discusses various selective additives, including carbon allotropes, ceramic materials, metallic materials, and multiple materials, to enhance tribological characteristics, biocompatibility, wear resistance, and mechanical strength. This review focuses on the fabrication of nano-HAps as coatings using selective deposition methods with controlled deposition parameters, paying special attention to recent developments in bone tissue engineering. This report is organized in such a way that it may inspire further research on surface modifications during medical treatment. The present review may help prospective investigators understand the importance of surface modifications for obtaining excellent implantation performance.